The present invention relates to an extrusion die for forming thin-walled honeycomb structures from extrudable materials such as glasses, glass-ceramics, ceramics, plastics, metals, cermets and other materials. In the ceramic arts, such dies are used for the extrusion of ceramics dispersed as powders in shapeable (plastic) extrusion batches to provide extruded green bodies of complex honeycomb shape.
Thin-walled ceramic honeycomb structures with multiple parallel through-channels or cells display utility in a variety of applications. For example, such structures exhibit utility as catalytic converters in the exhaust system of internal combustion engines. They also exhibit more general utility as catalyst carriers, filter bodies, and thermal regenerators or heat exchangers.
Dies used for the extrusion of ceramic honeycombs commonly have shallow, intercrossing and interconnecting slots on the downstream or exiting die face from which the ceramic batch emerges and which during emergence form the webs or sidewalls of the cells of the honeycomb structure being made. To supply the batch material to these slots, feed holes are provided in the opposite or upstream die face which connect with and feed batch material to the slots.
In common production dies the feed holes are aligned with the intersections of the slots on the outlet face of the die. This is because the intersections generally require larger proportions of the batch material for proper slot filling and web formation in the extruded shape. Some dies have a feed hole at every intersection, while other dies have holes at alternate intersections. Alternating hole patterns using fewer holes of larger diameter can be advantageous in that the dies are easier and less costly to produce, and are more resistant to bending deformation under high extrusion pressure.
Dies are also occasionally made with the feed holes aligned with the central portions of the slot segments, e.g., midway between the slot intersections. This feedhole positioning can improve the strength of the "pins", which are the projecting islands of metal bounded by the slots on the exit face of the die defining the channels in the honeycomb material extruded from the die.
Unexamined Japanese Patent Publication No. 50-151849 discloses an extrusion die having arrangement of feed holes and forming slots wherein the feed holes supply batch material principally to the longitudinal slot segments of the die rather than to the slot intersection portions. Unexamined Japanese Patent Publication No. 50-29922 describes extrusion dies for the continuous manufacture of ceramic honeycombs which comprise feed holes supplying either the slot intersections or the central portions of the slots.
A variety of die configurations for extruding honeycomb bodies of both triangular and square cell cross-section are known. U.S. Pat. No. 1,874,503, for example, discloses a triangular cell extrusion die wherein the feed holes supply batch material to the intersections of the triangular slots, this die being used for the extrusion of candy.
Dies of alternating feed hole design are also known. U.S. Pat. No. 4,741,792, for example, discloses a rectangular cell die configuration for extruding honeycomb ceramic heat exchanger bodies wherein the feed holes are positioned at alternating slot intersections. In this design, only two of the four corners of each extruded cell are formed by the direct flow of batch material thereto. The other corners of each cell are formed by lateral flow of the batch material within the slots to achieve the necessary web knitting at such other corners.
More complex arrangements of holes and slots are provided in compound dies comprising multiple body and/or face plate elements. For example, U.S. Pat. No. 4,243,370 describes a three-part honeycomb extrusion die comprising a slotted face plate, a feed hole plate, and a throttle plate, while U.S. Pat. No. 4,731,010 discloses a two-part extrusion die having a body plate and an abutting face plate, and wherein feed reservoirs for collecting and distributing batch material transmitted by the body plate are provided on the rear surface of the face plate.
Still other feed hole arrangements have been used when the honeycomb configuration of the extruded batch material is irregular. Thus published European Patent Application EP 0294106 describes extrusion dies for the manufacture of honeycomb-shaped ceramic regenerator bodies wherein feed holes of varying diameter are used to supply extrusion batch to the pin array forming the cells of the honeycomb. The variation is such that the largest feed holes supply batch material to regions of the honeycomb cross-section having the thinnest wall sections.
Notwithstanding these developments, no existing die design has proven adequate for the production of extruded honeycomb bodies of triangular cell cross-section with very high cell density. A specific problem not addressed in the prior art is that of achieving an adequate and uniform supply of extrudable batch material to a discharge slot array comprising a very large number of very fine slots. This is because, at higher and higher slot densities, more and smaller feedholes are generally required.
The principal difficulty encountered with these slot arrangements is that there is a practical minimum feedhole size, due principally to drilling technology limitations, which limits the density of the feedhole patterns available. Thus, even at minimum attainable feedhole sizes, a too close spacing of feedholes produces a weak die structure. In general, then, a feedhole pattern permitting the use of larger and/or more widely spaced feedholes provides both die fabrication and die performance advantages.
Accordingly, it is a principal object of the present invention to provide a novel extrusion die and method for using it which can produce extruded green ceramic honeycomb preforms of triangular cell cross-section and high cell density.
It is a further object of the invention to provide an extrusion die design incorporating a novel arrangement of slots and feedholes such that high-cell-density bodies with triangular cell cross-sections and thin cell walls can be efficiently produced.
It is a further object of the invention to provide an improved method for manufacturing an extruded ceramic honeycomb shape of triangular cell cross-section, high cell density, and low cell wall thickness.
Other objects of the invention will become apparent from the following description thereof.